A field programmable gate array (FPGA) is an integrated circuit that has programmable logic circuits and programmable routing circuits. The programmable logic and routing can be configured with a configuration bitstream that is loaded into the FPGA from an external source. An FPGA may also have a test mode that is used by the manufacturer of the FPGA to check that the FPGA is fully-functional before being sold, and also used to perform failure analysis on an FPGA returned from the field. Test mode often provides greater access and control over elements in the FPGA than the access and control provided to an ordinary user through the bitstream. An FPGA may contain a fuse that can be used to permanently disable test mode access or to prevent the programming or read-back of encryption keys.